Son of a prominent landowner, Boniface was born on July 31, 1936, in Ganthier, where he also spent his formative years. He attended the National School of Ganthier for primary education and later moved to Port-au-Prince for secondary studies at Lycée Alexandre Pétion. His childhood was filled with happiness, surrounded by an affectionate mother, doting grandmother, and a respectable father, along with seven adoring brothers and two sisters. Both at home and in school, the youngest of the Alexandres was well-behaved and managed to avoid punishment. However, his grandmother's restriction on contact with the outside world left a lasting impact, leading him to become a solitary individual with few friends.

After completing his secondary education, Boniface contemplated joining the Military Academy, which was recruiting at the time. With his tall stature and intelligence, he could have easily excelled in any test and been in the same promotion as distinguished individuals like Roland Chavannes and Acédius St-Louis. However, his parents vetoed the idea, stating they did not want a military career for him. He had to choose between medicine and law, ultimately opting for the latter. He enrolled at the Faculty of Law and Economics, earning his degree in 1962. He began his career at Cabinet Lamarre on Quai Street, practicing law until he decided to pursue a career as a magistrate.